Autonomous implanted capsules are referred to as “leadless capsules” to distinguish them from the electrodes or sensors placed at the distal end of a lead, which lead is traversed throughout its length by one or more electrical conductors connecting by galvanic conduction the electrode or the sensor to a generator connected at the opposite, proximal end, of the lead.
Such leadless capsules are, for example, described in U.S. Patent Pub. No. 2007/0088397 A1 and WO 2007/047681 A2 (Nanostim, Inc.) and U.S. Patent Pub. No. 2006/0136004 A1 (EBR Systems, Inc.).
These leadless capsules can be epicardial capsules, which are typically fixed to the outer wall of the heart, or endocardial capsules, which are typically fixed to the inside wall of a ventricular or atrial cavity, by means of a protruding anchoring helical screw, axially extending from the body of the capsule and designed to penetrate the heart tissue by screwing to the implantation site.
In one embodiment, a leadless capsule includes detection/stimulation circuitry to collect depolarization potentials of the myocardium and/or to apply pacing pulses to the site where the leadless capsule is located. The leadless capsule then includes an appropriate electrode, which can be included in an active part of the anchoring screw.
It can also incorporate one or more sensors for locally measuring the value of a parameter such as the oxygen level in the blood, the endocardial cardiac pressure, the acceleration of the heart wall, the acceleration of the patient as an indicator of activity, etc. Of course, the leadless capsules incorporate transmitter/receiver means for wireless communication, for the remote exchange of data.
The present invention is nevertheless not limited to a particular type of leadless capsule, and is equally applicable to any type of leadless capsule, regardless of its functional purpose.
Whatever the technique implemented, the signal processing inside the leadless capsule and the remote transmission of data into or out of the leadless capsule requires a non negligible energy supply as compared to the energy resources a leadless capsule can store. However, due to its autonomous nature, the leadless capsule can only use its own resources, such as an energy harvester circuit (responsive to the movement of the leadless capsule), associated with an integrated small buffer battery. The management of the available energy is thus a crucial point for the development of autonomous leadless capsules and their capabilities, especially their ability to have an integrated self-power supply system.
Various techniques of energy harvesting have been proposed, adapted to leadless autonomous implants. U.S. Patent Pub. No. 2006/0217776 A1, U.S. Pat. No. 3,456,134 A and WO 2007/149462 A2 describe systems using piezoelectric transducers directly transforming into electrical energy the movement of a mass resulting from the acceleration of the patient's organs or body. However, given the relatively low excitation frequencies (below 10 Hz), the excursions of the movements are relatively large, which does not allow a for significant miniaturization. In addition, since these excitations do not have stable specific frequencies, the piezoelectric generator cannot operate in a resonant mode, and thereby loses much of its effectiveness.
Other devices have been proposed to transform pressure changes occurring within the body into electricity, including changes in blood pressure or those resulting from the movements of the patient's diaphragm during breathing. This transformation is effected by means of a magnetic microgenerator, functioning as an alternator or as a dynamo, by variations in magnetic flux induced in a coil. Reference is made to U.S. Patent Pub. No. 2005/0256549 A1, GB 2350302 A, U.S. Patent Pub. Nos. 2008/0262562 A1 and 2007/0276444 A1. Due to the presence of moving parts, however, the complexity of the design of the mechanical and electrical parts and their relatively large volume effectively limit, the miniaturization and the overall reliability of such a generator. Moreover and most importantly, such a generator is inherently sensitive to external magnetic fields and is not compatible with the magnetic resonance imaging systems (MRI) because of the very high static magnetic fields generated by these systems, typically in the order of 0.5 to 3 T or more.
It also has been proposed to use an electrostatic transducer made of electrodes modeling a capacitor, for example, with a set of combs and interdigitated-counter combs. One of the electrodes is secured to a support fixed on the body of the case, the other being coupled to an oscillating mass called “seismic mass”. This mass is set in motion by movement of the entire system including the transducer, and it carries with it one of the electrodes of the transducer, which thus move relative to the other by a variation of the dielectric gap and/or of the facing surfaces of the two electrodes. If the capacitor is initially pre-loaded with an energy charge, or if the structure includes electrets (or electrets films) to maintain a continuous load, the capacity variation causes an energy increase in this capacitor that can be extracted by an electronic circuit and then stored in a buffer battery. The mechanical energy collected by the oscillating mass can thus almost entirely be converted into electrical energy in a single cycle. This technique is described, for example, by F. Peano and T. Tambosso, Design and Optimization of a MEMS Electret-Based Capacitive Energy Scavenger, Journal of Microelectromechanical Systems, 14 (3), 429-435, 2005, or S. Meninger et al. Vibration-to-Electric Energy Conversion, IEEE Transactions on Very Large Scale Integration (VLSI) Systems, vol. 9, no. 1, pp. 64-76, 2001. This type of transducer has the same drawbacks, however, as the piezoelectric transducers because of limitations imposed by the oscillating mass, both in terms of miniaturization (the seismic mass is relatively large) and efficiency with respect to the driving movements. Indeed, the relatively low excitation frequencies (below 10 Hz) involve relatively large excursions and/or a relatively high mass of the oscillating element, which does not allow a significant miniaturization.
Another known energy harvester system, without an oscillating weight, is disclosed by U.S. Patent Pub. No. 2009/021292A1. This document discloses an energy harvesting power system incorporated into an implantable capsule in which the housing body has a deformable element resulting from changes in pressure of the surrounding environment. The deformation of this element is transmitted to an electrostatic transducer directly converting the mechanical energy of deformation into electrical energy, which is then delivered to a power management and storage module powering the device with energy. Note that such a system does not need to be resonant or to contain magnetic elements. However, the system described utilizes pressure variations that result at least partly from mechanical forces applied to the capsule, under the effect of contact forces with the surrounding tissues or deformation thereof. Thus, in the case of a system that is fully submerged in a body fluid (for example such an energy harvesting system used in an intracardiac capsule blood pressure changes during rapid changes in the systole-diastole cycle), the slow variations of atmospheric pressure disrupt the operation of the energy harvesting system: indeed, as the capsule is strictly waterproof, its interior volume is initially at the pressure defined during manufacturing and the equilibrium point at rest of the deformable element is offset compared to the nominal rest position if the atmospheric pressure varies.